1. Field of the Invention
The invention relates to a perfected Metal Detector for passageways, to be used at the entrances of premises needing protection from the admission of armed persons and for other uses connected with the interception of metal objects.
2. Description of Prior Art
In existing passageway Metal Detectors, the transit of a metal body inside the supervised area causes variations in the signals induced on the receiver windings which depend on the path taken by this object.
If held in the same position, the object in transit will cause different induced signals as its transit path moves from the center of the passageway,--where the weakest signals are induced,--and approaches the lateral panels or columns in the proximity of which the signals induced reach a maximum strength.
This lack of uniformity in detection as the path of the object or objects varies is a serious drawback because, once established the strength of the induced signal of the smallest object detected under the worst conditions (that is along a path towards the center of the supervised passageway), every induced signal of equal or superior strength must be detected, and therefore those produced by much smaller objects than that established as having the minimum detection will not be detected, when in transit in the vicinity of the transducers housed in the sides of the passageway.
All this causes numerous false alarms that reduce notably the flow of people in the supervised passageway and could lead to the controllers reducing the sensitivity of the unit to cut down on the number of false alarms.
These disadvantages have been confronted with the invention subject of Italian Patent No. 1216946, in the name of Giovanni Manneschi, inventor of the current invention, in which reference is made to a single, disturbing metal body in transit in the supervised entrance. In accordance with this invention, two receiver transmitter units are used, with one unit's receiver and the other unit's transmitter placed on the same side of the passageway, so that, working alternately, the passage of the disturbing metal body induces two signals on the two receivers that are of a strength that is in proportion to the eccentricity of the object's path within the passageway.
This makes it possible for an analysis and control circuit, via the comparison of the received induced signals, to make the necessary corrections so that the effects of the metal body's passage are rendered virtually independent from the eccentricity of its path and thus heighten considerably the device's discriminatory powers. This system therefore provides a correct response in every case of a single metal object in transit; on the other hand, in the case of the simultaneous passage of two metal bodies, one close to one panel, the other close to the opposite panel, or anyway at a close distance from one transducer section and the other from the other section, regardless of the structure and arrangement of these sections, the system gives surprising signal variations in signal on the receiver windings that are virtually identical to each other and in size are equal to the induced signal caused by the passage of a single metal object that is, however, much larger than the two preceding ones, in transit in the center of the supervised entrance.
Therefore, the simultaneous passage of two small metal bodies, such as keys and/or coins, along two paths next to transducers placed on the sides of the supervised passage, produces an effect comparable to that produced by a weapon in transit along a path in the center of the same supervised passage.
In the case of the simultaneous transit of two bodies, the invention described in Pat. No. 1,216,946 is therefore unable to interpret the phenomenum correctly and provide a correction of the reception sensitivity of the single receiver windings because, unable to comprehend the phenomenum, if the correction was to take place it would also be applied to the passage of a single mass in the center of the entrance.